290 Brown . — Shcdies in the Physiology of Parasitism. IX. 
obvious at a glance. With young vigorous spores which may give an 
average germ-tube length of two to four divisions of the micrometer scale in 
the control tests, the result is not so obvious. On the other hand, spores of 
vigorous germinative capacity are desirable where a retarding effect is con- 
cerned. In many experiments, therefore, the practice has been to use both 
vigorous aftd feeble spores, so that suitable conditions were present for 
detecting both retarding and stimulating effects. 
The vigorous type of spore is obtained from 1-2 week old cultures 
on potato agar : the feebler spores from cultures of 4-8 weeks’ age. In 
very old cultures still feebler germination is obtained, but the individual 
variations among the spores of a culture become more and more pronounced, 
not only when the spores are sown in water, but also when they are 
germinated in a good nutrient. This lack of uniformity places a limit 
beyond which the spores cannot be profitably used. 
A method that proved ‘useful for ensuring small germination in the 
control tests was to make use of the retardation due to blotting-paper 
already mentioned ; this point will be illustrated later. 
Apart from the effect of stimulation which results in an increased 
average germ-tube length, a distinct formative effect has been noted in many 
cases. This takes the form of increased stoutness of the germ-tube. In 
such cases it is not possible to give an adequate quantitative expression of 
the result. The stimulated and the control spores may on measurement 
give very much the same average germ-tube length, though the appearances 
presented in the two cases may be very different. In some cases, as will be 
noted subsequently, it is not possible to describe the effect either as a 
simple stimulation or a retardation, since the control germ-tubes take the 
form of long thin hyphae, whereas those formed in the presence of the plant 
tissue are shorter but much stouter. 
Experiments have been carried out showing that the volatile stimulants 
are sufficient to increase markedly the parasitic powers of Botrytis spores. 
An experiment along these lines gave the results shown in Table II. 
The experiment was set up in the general way already described, with 
the difference that several leaves of broad bean were put in, each leaf having 
on it a number of drops of the same spore suspension as was used for study- 
ing the amount of germination on the glass slides. The figures in the last 
column are in the form of fractions, the denominator of which gives the 
number of inoculations, the numerator the number of infections which had 
taken on the second day from sowing, at which time all the readings in the 
table were taken. It may be remarked that the increased parasitic effect 
was greater than the figures would indicate, as the greater percentage of 
infections in presence of the plant tissue was accompanied, on the average, 
by a much more advanced attack. 
The effect produced by bruised bean leaves was only got by adding an 
